Exhaust mufflers often include a sound absorbing material within the interior of the muffler to dampen the sound made by the escaping gases that pass through the muffler. Many types of exhaust mufflers are produced by mechanically joining multiple pieces to form a muffler shell. For example, a common type of exhaust muffler is a clamshell muffler, which is assembled by joining an upper section to a lower section by welding or crimping. Another common construction for mufflers uses a single body portion that is sealed by joining end portions to the body portion by welding or crimping. A common material used to fill exhaust mufflers is glass fibers. The fibers usually fill at least part of the interior muffler cavity, and are often inserted into the muffler in a volumized form that makes them somewhat difficult to contain. Often, when volumized fibers are used, fibers stray from the interior muffler cavity and become trapped between the sections of the muffler shell. The trapped fibers subsequently have an adverse effect on the quality of the joint between the sections of the muffler. Hence, there is a need for an improved muffler filling process.